1. M. Maruyama, K. Tsukada, K. Ozawra, F. Fujimoto, K. Komaki, M. Mannami andT. Sakurai:Nucl. Phys.,145 A, 581 (1970).
2. G. J. Clark, J. M. Poate, E. Fuschini, C. Maroni, I. G. Massa, A. Uguzzoni andE. Verondini: to be published inNucl. Phys., and A.E.R.E. Report R. 6756 (1971).
3. I. G. Massa: Report INFN/BE-69/9 (1969).
4. A sharp cut-off distance beyond which no blocking is evident has been suggested byJ. Lindhard:Mat. Fys. Medd. Dan. Vid. Selsk.,34, No. 14 (1965).
5. P. Mermier andE. Sheldon:Physics of Nuclei and Particles, Vol.2, Chap. 14 (New York and London, 1970).